FRONTLINE investigates the miscalculations and mistakes behind the brutal rise of ISIS. As part of a special FRONTLINE series, Correspondent Martin Smith reports from Iraq on how the country began coming undone after the American withdrawal and what it means for the U.S. to be fighting there again.

Public health scientists and clinicians tout vaccines as one of the greatest achievements of modern medicine. But for many ordinary Americans vaccines have become controversial. Young parents are concerned at the sheer number of shots -- some 26 inoculations for 14 different diseases by age six -- and follow alternative vaccination schedules advocated by gurus like Dr. Robert Sears. Other parents go further. In communities like Ashland, Oregon, up to one-third of parents are choosing not to vaccinate their kids at all. And some advocacy groups, like Generation Rescue, argue that vaccines are no longer a public health miracle but a scourge; they view vaccines as responsible for alarming rises in certain disorders, including ADHD and autism. This is the vaccine war: On one side sits scientific medicine and the public health establishment; on the other a populist coalition of parents, celebrities (like Jenny McCarthy), politicians and activists. It's a war that increasingly takes place on the Internet with both sides using the latest social media tools, including Facebook and Twitter, to win the hearts and minds of the public.

The Interrupters presents profiles in courage, as three former street criminals in Chicago place themselves in the line of fire to protect their communities. The film follows the lives of these "Violence Interrupters," who include the charismatic daughter of one of the city's most notorious former gang leaders, the son of a murdered father, and a man haunted by a killing he committed as a teenager.

As they intervene in disputes to prevent violence, they reveal their own inspired journeys of struggle and redemption. From director Steve James (Hoop Dreams) and bestselling author Alex Kotlowitz (There Are No Children Here), "The Interrupters" chronicles an intimate, year-long journey across the stubbornly violent landscape of our cities through the eyes of those fighting to sow peace and security.